Various polymers exhibiting optical anisotropy in a molten state (liquid-crystal polymers) have been proposed as thermoplastic resins which are excellent in both heat resistance and processability. Representative examples thereof are polymers disclosed in commonly assigned U.S. Ser. Nos. (1) 335,444, filed Apr. 10, 1989, (now U.S. Pat. No. 4,918,154) (2) 335,438 filed Apr. 10, 1989, (now U.S. Pat. No. 4,937,310), and (3) 335,504, filed Apr. 10, 1989 (now U.S. Pat. No. 4,920,197). These liquid-crystal polymers each exhibit liquid-crystal properties by virtue of the rigid segment incorporated into its skeleton, so that they are excellent in strength and heat resistance and are easily processable by virtue of their excellent melt flowability.
However, it is important for achieving the practical use of a liquid-crystal polymer in some industrial fields that it be further improved in the areas of heat resistance and processability as discussed hereafter.
That is, with respect to processability, a thermoplastic resin should have a suitable flow-initiating temperature, flowability in a molten state, and should be sufficiently thermally stable to cause neither excessive generation of gas nor discoloration during its processing.
Among them, the flow-initiating temperature and the flowability can be improved by utilizing a liquid-crystal polymer. However, when the processing of a thermoplastic resin is carried out at a temperature exceeding 300.degree. C., it is difficult to inhibit by the addition of a conventional stabilizer for thermoplastic polymers the generation of gas resulting from polymer decomposition and discoloration during processing. Accordingly, the processing temperature of a thermoplastic resin commonly cannot be too highly elevated.
The preferred molding temperature for a thermoplastic resin, particularly a polyester, is about 370.degree. C. or below. At a temperature exceeding 370.degree. C., the thermal degradation of a thermoplastic resin commonly proceeds rapidly, so that no consistent strength of a thermoplastic resin can be guaranteed following such thermal processing. Further, such a processing temperature is beyond the service temperature range of a conventional molding machine.
With respect to heat resistance, a thermoplastic resin should be excellent in retention of form and mechanical strengths in a high-temperature atmosphere. Particularly, a thermoplastic resin to be used in an electric or electronic field is inevitably subjected to soldering, so that the stability of form and physical properties thereof at 260.degree. C. or above is more important.
From the standpoints described above, a liquid-crystal polymer should also be improved in the areas of both processability and heat resistance. However, these two characteristics tend to be contradictory to each other for conventional thermoplastic resins, so that it is very difficult to lower the melting point or pour point of a liquid-crystal polymer, which is an indication of the processability thereof, while enhancing the reliability of heat distortion resistance and mechanical properties thereof at an elevated temperature.
A liquid-crystal polyester derived from 6-hydroxy-2-naphthoic acid p-hydroxy benzoic acid, aromatic diol and aromatic diacid (as defined therein) in certain concentrations is disclosed in U.S. Pat. No. 4,219,461.